2206
D. BEHAR,G. CZAPSKI,AND I. DUCHOVNY
Carbonate Radical in Flash Photolysis and Pulse Radiolysis of Aqueous Carbonate Solutions by David Behar, Gideon Czapski, and Itzhak Duchovny Department of Physical Chemistry, Hebrew University, Jerusalem, Israel
(Received October 7, 1069)
The carbonate radical, co,-, may be generated by the reaction of carbonate or bicarbonate ions with hydroxyl radicals. The flash photolysis of hydrogen peroxide solutions and the pulse radiolysis of water are utilized as sources of hydroxyl radicals from which it is found that the basic form of the hydroxyl radical, 0-, contributes little to the production of cos-, even up to pH 14. Reactions of C0,- with hydrogen peroxide ( k = 8 x lo5 M-' sec-1) and with HOz- ( k = 5.6 X lo7 iMV1 sec-l) are observed. The carbonate radical and the perhydroxyl radical ion, 02-,are formed in equimolar concentrations in the flash photolysis of oxygensaturated carbonate solutions, and from this, the extinction coefficient of 02- is found to be 1850 f 200 cm-l by comparison with the better known extinction coefficient of co,-. The product of the reaction COS02- is assumed to be CO,2- having a half-life time of several seconds and E C O ~ = ~ 410 - ~ ~M-l ~ cm-l.
+
Introduction The carbonate radical ion, produced in pulse radiolysis of aqueous solutions, is assumed to be formed according to the reactions'-s OH
+ COS'- +COS- + OH-
(la)
I n highly alkaline solutions, 0- is assumed to react with carbonate to give the same r a d i ~ a l according ,~ to
+ C032- % COS- + 20H-
0-
(1b)
At relatively low pH's, reaction l a may be replaced by OH
+ HCO3- +COS- + H20
(IC)
The spectrum of the radical was obtained4p5with a maximum absorption a t 6000 8. The same spectrum was found when the radical was produced by flash photolysis of carbonate solutionss according to ~
0
~
-% 2 - CO,-
+ e,, -
(2)
The decay of COS- in oxygen-free solutions is found to be second order, according to one of the following reactions6
cos- + CO3- --+ coz 4 co42-
(3a)
or C0,-
+ C0,-
-%2C02 + HOz-
+ OH-
(3b)
I n flash photolysis and pulse radiolysis of 02-satare produced. urated carbonate solutions, CO3- and 02I n these systems, COS- decays mainly in the reaction COB-
+ 02-+products
(4)
The rate constant of this reaction is a subject of disagreement in the literat~re.~!'We studied the reactivity of OH and 0- toward COS^-, as well as that of Cor- toward H202as a function of pH. The reaction T h e Journal of Physical ChemktTy
of COS- with 02- and the extinction coefficient of 0 2 - were also studied.
Experimental Section Materials. NaHCO3 and r\ia2C03 (Baker) and NaOH (Merck) were of analytical grade and were used without further purification. Hydrogen peroxide (BDH) was purified by irradiation, followed by distillation collecting the middle fraction only. This procedure was twice repeated. All water used was triply distilled. Sample Preparation. Solutions were saturated with N a or O2 in syringes. The solutions were introduced from syringes into the irradiation cell, through A5 ground joints. The details of this method have been described previously.* Analytical Methods. The concentration of the hydrogen peroxide was determined by titration with potassium permanganate. The proper concentrations were made by dilution, a few seconds before the flash, so that the thermal decomposition of H2Oz could be ignored. Apparatus. Samples were irradiated in Spectrosil cells. The cell for flash photolysis was 5 cm long and 1.4 cm i.d. Details of the flash photolysis assembly have been described previously.8 Some solutions (1) S. Gordon, E. J. Hart, M. S. Matheson, J. Rabani, and J. K. Thomas, J . Amer. Chem. SOC., 85, 1375 (1963). (2) G. E. Adams and J. W. Boag, Proc. Chem. Soc., 112 (1964). (3) G. E. Adams, J. W. Boag, and B. D. Michael, Trans. Faraday
SOC.,61, 1417 (1965). (4) G. E. Adams, J. W. Boag, and B. D. Michael, ibid., 61, 1674 (1965). (5) J . L. Weeks and J. Rabani, J . Phys. Chem., 70, 2100 (1966). (6) E. Hayon and J. J. McGarvey, ibid., 71, 1472 (1967). (7) G. E. Adams, J. W. Boag, and B. D. Michael, Proc. Roy. Soc., A , 289, 321 (1965). (8) D. Behar and G. Czapski, Isr. J . Chem., 6, 43 (1968).
COa RADICAL IN FLASH PHOTOLYSIS AND PULSE RADIOLYSIS
2207
were irradiated by pulse radiolysis in a 4-cm cell, with light passing thrice through the cell; details of this technique will appear el~ewhere.~Pulses (200 mA) of 5-MeV electrons with pulse duration of about 1 psec were used.
Results and Discussion The hydroxyl radical was produced in flash photolysis of hydrogen peroxide solutions
52 0 H
(5)
HzOz
and produces COS- by some combination of reactions l a , lb, and IC. In the presence of hydrogen peroxide, another reaction of the hydroxyl radical is
“OH”
+ “H202”
--f
“HOz”
+ HzO
~ , , O H ” +c
o p as Function of
[OH-]
[OH-], M
5.9 1.2 0.1 10-8
450
I KK)
I
550
(6)
As all experiments were carried out at pH >8, HO2 was totally ionized and present as Oz-.’O “OH” and “Hz02” may be present in their dissociated or undissociated forms (depending on the pH of the solution). In order to work under conditions in which reaction 6 can be ignored in comparison to reactions l a and lb, we redetermined the rate constant of “OH” with COS2- as a function of pH. The Reactivity of “OH” with COS’-. The reaction of “OH” with C 0 2 - was measured in pulse radiolysis of carbonate solution saturated with N2O at pH 1114.8. The formation of COS- was followed at 6000 8 and found to be of first order. The pseudo-first-order rate constant kobsd was determined at each pH for 4-5 different concentrations of C032- (at pH 14.8 for one COS^- concentration.) The second-order rate constant was determined as the slope of the line of kobsd us. carbonate concentration. The results are given in Table I.
Table I : Dependence of
01
1.1 x 106 2 . 5 X lo6 4.8 x 107 3 . 6 5 X 108
The value of the rate constant at pH 11 is that of
kl, which is in good agreement with earlier determinations (3 X los M-’ sec-’,” 2 X lo8 M-’ sec-’,3 4.2 X 108 M-’sec-l.5 The rate of k l b was given as 4.4 X 107 M-’ sec-1,12 < l o 7 M-’ ~ e c - ’ . ~Our determination (Table I) shows that most of the reactivity of “OH” even up to pH 14 could be due to the small fraction of the undissociated hydroxyl radical. If 0- reacts with COa2- a t all, we find k l b 5 5 X lo6 M-’ sec-l which is smaller by about
I 650
I
500
I m
I
750
I
my
Figure 1. Spectrum of COa- radical formed in flash photolysis of Nz-saturated solution of HZOZwith N a ~ C 0 3or NaHC03: M A, 2 X 10-4 M HzOz 1 M NaZCO3,pH 13; 0, HzOz 1 M NaHC03, p H 8.
+
+
two orders of magnitude as compared to earlier determinations. In the flash photolysis experiments, even a t the highest pH (pH 14) the ratio [COS~-]/[HO~-] was Table 2500 or greater, the value of ~ , , O H , , + C O ~ %from I is 2.5 X lo6M-’ sec-’, and k8b = 5 X lo8M-‘ sec-‘,s
+ HzO2 +HOz + HzO 0- + HOz- + + OHOH
02-
(64 (6b)
so the ratio k t , ~ ~ ” + ~[C032-]/ks~ osl[HOS-] is 12.5. In most experiments the ratio mas much higher as both, [HzOz]was smaller and the pH was lower. It is clear that under these conditions, reactions 6a and 6b do not occur to any appreciable extent and that any Oz- formed through reaction 6 would be negligible. Spectrum of COS-. The shape of the absorption spectrum of the carbonate radical was determined in flash photolysis of H202,in the presence of KaHC03 or XaZCO3,at pH 8 and 13, respectively. The spectra are identical within experimental error at these pH’s (Figure 1). There is no evidence that two forms of the COS- radical might exist in the pH range 8-13, due to any acid-base equilibrium. This is in accordance with the pH independence of the recombination rate constant of the carbonate radical, apart from the effect of the ionic strength.5 Earlier determinations of the absorption spectrum5t6 agree with ours, having the absorption maximum a t (9) Internal Report of the Accelerator Laboratory, Hebrew University, Jerusalem, Israel. (10) G. Ceapski and L. M. Dorfman, J. Phys. Chem., 68, 1169 (1964). (11) J. K.Thomas, Tram. Faraday SOL, 61, 702 (1965). (12) M.Anbar and P. Neta, Intr. J. A p p l . Radiat. Isotop., 18, 493 (1967). Unpublished data of D. M. Brown. Volume 74, Number 10 M a y 14, 1970
D. BEHAR,G. CZAPSKI, AND I. DUCHOVNY
2208
1
l
+
where (“HzOZ”) = (HzOZ) (HOZ-). Equation 7 leads us to assume the existence of reaction 8. As the COS-
+ “H2OZ”+products
(8)
“Hz02” concentration was much higher than that of decay of COS- was to be expected. First-order plots, at various HzOz concentrations, are given in Figure 2; the pseudo-first-order rate constants were calculated from these. The kobsd values thus obtained were then plotted as a function of HzOzconcentration (Figure 3) and ks was calculated. Reaction 8 explains the deviations from the secondorder plots of the COa- recombination kinetics, found by Weeks and Rabani.s (They found that the secondorder decay was too fast as the reaction proceeds.) If recombination of COS- produces HzOz, (reaction 3b), the decay of COS- would be self-catalytic. The second-order rate constant k8 is found to be pH dependent. Figure 4 shows the variation of k8 as a function of pH. Such a behavior is typical of a system where one (or both) of the reactants have two forms which are in acid-base equilibrium and which react with different rates. However, the possibility that the carbonate radical is in an acid-base equilibrium
cox-, a first-order
el-
\‘
t Jlscc
Figure 2. Typical decay curves of COS- followed a t 6000 b, in flash photolysis of Nt-saturated solution of HtOt and 1 iCf NazCOa, a t p H 11.2: 0, 0.11 mM HzOt; A, 0.17 mM HzOZ; x, 0.22 mM HzOt; A, 0.33 mM HzOt; ,. 0.44 mM H z O ~ ; 0 , 0.55 mM H ~ O Z0, ; 0.66 mM H z O ~ . 20000
I I I
H+ 15000
‘3 B
HC03
in the pH region studied can be ruled out because of the following reasons: (a) the pH independence of the recombination rate constant of the COa- r a d i ~ a l ; ~ (b) the identical spectra of the radical when measured at pH 8 and 13; (c) the dissociation constant of the second hydrogen of carbonic acid is 4.4 X (pK = 10.36). The pK of the radical would be ex-
10000
. I
500C
C
+ co3-
I
0.4
0.2
0.6
p*
021 mt.4
Figure 3. The dependence of the pseudo-first-order rate constant of the reaction of COS- with HzOZ,on [HzOZ] . (Values of kobsd are obtained from Figure 2.)
6000 1. We did not, however, determine the absolute value of the extinction coefficient = 1860 cm-’ LW-l, found by Weeks and Rabani).5 The Reaction of COS- with HzOz. The decay of the COS- absorption was followed at 6000 A in the flash photolysis of HzOz in solutions containing 1 mol/l. of c03’- and HC03-. The decay was first order in both COS- and HZ01 and followed the rate equation The Journal of Physical Chemistry
I
I
I
COSRADICAL I N FLASH PHOTOLYSlS
AND PULSE
2209
RADIOLYSIS
pected to be lower than this (similar behavior is seen in the cases of H202:H0210913,14 and ROH:R016). As COS- is probably not responsible for the change of ks with pH, one comes to the conclusion that the equilibrium H202 a H + H02- is the cause of the pH dependence of ICs. We suggest that the following mechanism governs the disappearance of the carbonate radical in reaction with hydrogen peroxide COS- H202 -+ products (9)
+
+ COS- + HOzproducts K HzOz Jr H + + HOz4
13
12
5 11
(94
where K is the dissociation constant of HzO2. Thus
la
+ (HOz-) the decay rate will be
If (“H202”)= (H202)
9
K
+
[H+]
-2
-1
I
I
0
1
Figure 5. The dependence of pH on a function of
kobsd
and
p.
with dependence of k9, was neglected as p was almost constant, and about 3 in all experiments. The Reaction C 0 3 02-. As we wanted to follow the reaction which takes place between the carbonate radical and the hydroperoxy radical, we flash photolyzed a 0.2 M NazC03 solution saturated with oxygen. The primary photolytic process is probably
+
Rearranging eq 10, one obtains
+ e,, -
C O ~ Z-% - CO~-
(2)
followed by
or pH = pK
+ log
kobsd
kga
-
- k9 kobsd
where K is the equilibrium constant at the measured ionic strength. With a correction made for the ionic strength,I4eq 11 can be expressed as
where A = 0.5, p is the ionic strength, and K Ois the equilibrium constant a t zero ionic strength. A plot of PH 0s. log (kobsd - h / k 9 a - kobsd) - (AV$/l &) should yield a straight line, with intercept equal to pKo. When taking the plateau values of kobsd as given in Figure 4 (kg = 8 X lo6 M-l sec-l, and kga = 5.6 X lo7 M-’ sec-l) and using eq 12, we did obtain a straight line which is shown in Figure 5 , and from which we derived the value of pKo = 11.7 f 0.2. This value agrees very well with that of 11.85 f 0.1 obtained by Jortner and Stein14a t 19”, and of 11.76 f 0.02 obtained by Evans and Uri16at 20’. The possible ionic strength
ea,-
+ 02 -+ 02-
(13)
There are practically equal concentrations of COSand 02-at the end of the flash. In addition to reaction 2, some of the hydroxide ions may be photolyzed
OH-
-%
OH
+ eaq-
(14)
This would have no effect on our assumption of equimolar concentrations of 0 2 - and Goa-, as eaqwill yield 02-and OH will form with carbonate COS-. We found the absorption to decay at 6000 in a second-order process, but a t 2600 A the absorption had two decay modes: second order in the millisecond range and a first-order one with a half lifetime of about 2 sec. This slow decay is similar to that of 02-in the alkaline region. If 02and COS- concentrations (13) H.A. Schwarr, J. Phys. Chem., 66, 255 (1962). (14) J. Jortner and G. Stein, Bull. Res. Counc. Isr., A , 6,239 (1957). (15) K. D. Asmus, A. Henglein, A. Wigger, and G. Beck, Ber. Bunsenges. Phys. Chem., 70, 756 (1966). (16) M.G. Evans and N. Uri, Trans. Faraday Soc., 45, 224 (1949). Volume 74, Number 10 May 14, 1970
D. BEHAR,G. CZAPSKI, AND I. DUCHOVNY
2210 are equal, then it is not very probable that any excess of 02-will be left a t the end of the second-order decay, as the reaction of COS- with 0 2 - is much faster than those of 02- 0 2 - and COaGoa- a t the same pH. I n order to make sure that the long-lived intermediate was not the 02-,we pulse-radiolized a 0.2 M solution of carbonate containing O2and N20,in which the initial ratio after the pulse of COS- to 0 2 - amounted to 3.8. We found that this long-lived intermediate exists even in this solution, and the ratio Do2600/D_2600 remained 4 in the presence, and absence, of N20, which is inconsistent with its identification as excess of 02-. (Do2600is the initial absorption a t 2600 A and Dm2600is the absorption taken 5 msec after the pulse, where the fast decay is over and no appreciable decay of the long-lived product occurs.) Thus, the formation of a n e \ - intermediate is suggested Lvhich might be the COS2- (the recombination product of COS02-)and which decomposes most probably in a first-order process to C03*02. From the initial absorption of 02a t 2600 A, taking e ~ = 1850 ~ M-' em-', ~ ~ me de~ termined E C O ~ %=- ~(410 ~ ~ ~& 40) M-' cm-'. A similar behavior was observed by Hayon'' in flash photolysis of aerated phosphate solutions where 02-and H2P04 (or HP04-) were produced simultaneously and, after the 0 2 - decayed, a long-lived intermediate waso observed with a niaximum absorption around 2600 A. was determined from the results of the flash photolysis of oxygenated carbonate solutions, where equimolar concentrations of CO1- and 02-are formed. EO^-^^^^ was calculated from the initial absorptions of 02-and COS- at 2600 8 and that of COS- at 6000 8, taking C . C O ~ - ~ " O O = 1860 M-' cm-I5 and E C O ~ - =~ ~ 200 M - I cm-I.5 We obtained ~ 0 ~ - ~ 6 0=0 (1850 200) M - I cm--I. This value is compared with other determinations in Table II.7t10318-21 The decays of 02-and C03- at 2600 8 and 6000 8 in the flash photolysis of 02-saturated Co32-solutions was fo>ind to be second order. A t 2600 8 1/(D2600 - Dm260O)was plotted as a function of time and the
+
The Journal of Physical Chemistry
A,
+
+
+
Table 11: The Extinction Coefficient of 02A
2540 2600 2600 2600 neutral pH 2600 alkaline pII 2600 2600 2600
1900 870" 1220 1000 1100-2000 1675 1700 1850
18 10 7 19 19 20 21 This work
' This value is not accurate as no correction was made for the absorption of the OH radical.
+
slope of the line was taken as k4/1(~o,-2600 ~ ~ ~ ~ - 2-6 0 E C O ~ ~ - ~ Using ~ ~ ~ ) . the extinction coefficients of O,;, COS-, and COj2- a t 2600 8 and that of C03- a t 6000 A (1850, 200, 410, and 1860 M-' cm-l, respectively), we obtained at both wavelengths the same rate con~stant for the reaction
COa-
+ 02-+products
(4)
(4 f. 1) X lo8 144-' sec-I in comparison with 1.5 X lo9 M-' sec-I reported by Adams, et aLJ7and sec-I 1.4 X lo8 AI-' sec-I (the value 1.4 X lo8 A'-! was calculated assuming EO^-^^^^ = 900 M-' cm-I, taking the value of t02-2600= 1850 M-' cm- found by us one arrives at kr = 2.8 X lo8 M-' sec-l) by Hayon and hIcGarvey.6 1 ~ 4=
Acknozdedgment. We gratefully acknowledge the support of this research by the U. 5. Atomic Energy Commission under Contract AT(30-1) -3753. ~ ~
(17) J. Robert Huber and E. Hayon, J . Phus. Chem., 72, 3820 (1968). (18) J. H. Baxendale, Radiat. Res., 17, 312 (1962). (19) J. Rabani, Advances in Chemistry Series, KO.81, American Chemical Society, Washington, D. C., 1968, p 131. (20) J. Rabani and S. 0. Nielsen, J . Phys. Chem., 73, 3736 (1969). (21) D. Behar, G. Csapski, L. M. Dorfman, J. Rabani, and H. A . Schwars, t o be published.
0
